Son returns with numbers (3)
I believe the answer is:
nos
'numbers' is the definition.
(no. is a kind of number)
'son returns' is the wordplay.
'returns' says the letters should be written in reverse.
'son' backwards is 'NOS'.
'with' acts as a link.
(Other definitions for nos that I've seen before include "Numbers (abbr.)" , "they opposed" , "Issue back > numbers" , "Numbers (abbreviation)" , "Numbers (abbrev.)" .)